I have previous experience using Bukkit. My BukkitDev alias is 'x3nec' and I have worked on plugins such as MoxieSkills, DragonTravel, Heroes (private version for HeroCraft) and LiveChat.
As well as creating the plugins themselves, I also try to add API and documentation where I can. This means that either I, or another developer, could easily integrate into the systems without too much additional work. I consider this a personal requirement in all of my projects.
I have a tester contracted to me, at no additional cost to yourself. Therefore, development and testing can be done at the same time, reducing my turn-around time for projects. Initially I would collect all key points for the systems. Then I would develop them, at which time they would be tested. Then I would see that a sufficient API is implemented to future-proof the system.
Whilst more information would be needed to give n accurate plan, my basic course of action for plugins is as follows:
1) Create a requirements specification - something that both you and I would agree on as the basic minimum required from the system.
2) Begin development of the system. Simultaneously, new features will be tested by my tester, so as to avoid additional paid hours to the client in performing this task (basically giving a quicker turnaround).
3) Once the system is complete, I would implement API features and other support features, to make it "future proof" for either myself or other plugin developers.
ETA: 10 days maximum